Vegetation-center of the Prairies. 237 
Superior. Another area covered by silicious sandstone is the pine district of 
the state, while south of Wisconsin is the region of oak openings and prairies. 
And when we reach these treeless tracts, we find we have gotten entirely 
beyond the drift covered areas and that we are upon a soil made up of the 
insoluble residuum left from the disintegration of several hundreds of feet of 
limestone and dolomites. 
e vegetation-center of the prairies is found in Nebraska, Iowa, 
Kansas and the Dakotas. From this center the typic plains flora slowly 
shades out toward both north and south. All vegetation regions, and none 
more readily than a plains region, where dissemination is so easily affected, 
borrow floral elements from adjacent regions, and it is only when this invasion 
has resulted to a pronounced degree that the original floral covering changes 
aspect. It is to the elucidation of these various elements in the plains flora 
that we must now direct our attention. 
One of the most striking elements is the intrusion into the prairie region 
of trees and shrubs which have migrated from the northeast along the water 
courses emptying into the Missouri and Mississippi rivers. The trees which 
have entered Nebraska in this way according to BESSEY”') are Juniperus vir- 
giniana, Asımina triloba, Salix (3 species, 6 in number), Populus monilifera 
(= PP. deltoides), Tilia americana, Ulmus americana, U. racemosa, U. fulva, Celtis 
occidentalis, Morus rubra, Fraxinus americana, F. pubescens, F. viridis, Pyrus 
(Malus) coronaria ioensis, Crataegus (4 species), Amelanchier canadensis, Prunus 
virginiana, Prunus serotina, Gymnocladus canadensis, Gleditschia triacanthos, 
Cırcis canadensis, Platanus occidentalis, Rhamnus ERROR R. caroliniana, 
Aesculus glabra, Negundo aceroides Eile negundo), Rhus copallina, Fuglans 
cinerea, F. nigra, Hicoria (Carya, 5 species), Ouercus (9—ı0o species), Osirya 
virginiana, Carpinus caroliniana, and possibly one birch. Here are fifty-six 
or fifty-seven species of trees, which have without question come into the 
region from the forests of the southeast. 
The trees which have entered the prairie region from the Rocky Mountain 
forests have made much less impression. Those which belong to this list are 
Pinus ponderosa var. scopulorum, Funiperus scopulorum, Populus (4 species), 
Shepherdia argentea, Acer glabrum and Betula (2 species). 
Occasionally these eastern and western floral elements meet. 
BESSEY mentions a case). Long Pine Creek runs northward about twenty 
miles to the Niobrara River through a deep and winding canyon. In this 
canyon, a blending of the eastern and western floras takes place. Pinus ponde- 
rosa var. scopulorum occurs in the greatest abundance here, but not eastward 
of this stream, while the black walnut Fuglans nigra of the east is found as- 
sociated with the bull pine, as also Os/rya virginiana, which with Prunus 
ı) BEssey, C. E.: The Forests and forest Trees of Nebraska. Annual Report Nebraska State 
Board Agriculture 1899: 8182; Plant Migration Studies, University of Nebraska Studies V, No. r. 
Jan. 1905. 
2) BEssey, C. E.: A Meeting-place for two Floras. Bulletin Torrey Botanical Club XIV: 189. 
